There are three types of electrolytic cells commercially used for producing halogen gas and aqueous caustic solutions from alkali metal halide brines, a process referred to by industry as a chlor-alkali process. The three types of cells are: (1) a mercury cell, (2) a diaphragm cell and (3) a membrane cell. The general operation of each cell is known to those skilled in the art and is discussed in Volume 1 of the Kirk-Othmer Encyclopedia of Chemical Technology, (John Wiley & Sons-3rd Ed. 1978) at page 799 et. seq., the relevant portions of which are incorporated herein by reference.
The three cells differ in various respects. In the mercury cell, alkali metal ions produced by electrolysis of an alkali metal salt form an amalgam with mercury. The amalgam reacts with water to produce aqueous sodium hydroxide, hydrogen gas and free mercury. The mercury is recovered and recycled for further use as a liquid cathode. In a diaphragm cell, an alkali metal halide brine solution is fed into an anolyte compartment where halide ions are oxidized to produce halogen gas. Alkali metal ions migrate into a catholyte compartment through a hydraulically-permeable microporous diaphragm disposed between the anolyte compartment and the catholyte compartment. Hydrogen gas and aqueous alkali metal hydroxide solutions are produced at the cathode. Due to the hydraulically-permeable diaphragm, brine may flow into the catholyte compartment and mix with the alkali metal hydroxide solution. A membrane cell functions similar to a diaphragm cell, except that the diaphragm is replaced by a hydraulically-impermeable, cationically-permselective membrane which selectively permits passage of hydrated alkali metal ions to the catholyte compartment. A membrane cell produces aqueous alkali metal hydroxide solutions essentially uncontaminated with brine. Presently, the most widely used chlor-alkali processes employ either diaphragm or membrane cells.
The minimum voltage required to electrolyze sodium chloride brine into chlorine gas, hydrogen gas, and an aqueous sodium hydroxide solution may be theoretically calculated by the use of thermodynamic data. However, in reality, production at the theoretical voltage is not attainable and a higher voltage, i.e., a so-called overvoltage, must be applied to overcome various inherent resistances within the cell. Reduction in the amount of applied overvoltage leads to a significant savings of energy costs associated with cell operation. A reduction of even as little as 0.05 volts in the applied overvoltage translates to significant energy savings when processing multimillion-ton quantities of brine. As a result, it is desirable to discover methods which will minimize overvoltage requirements.
Throughout the development of chlor-alkali technology, various methods have been proposed to reduce the overvoltage requirements. To decrease the overvoltage in a diaphragm or a membrane cell, one may attempt to reduce electrode overvoltages, i.e., a so-called hydrogen overvoltage at the cathode; to reduce electrical resistance of the diaphragm or membrane; to reduce electrical resistance of the brine being electrolyzed; or to use a combination of these approaches. Some research concentrates on minimizing cell overvoltage by proposing design modifications to the cells.
It is known that the overvoltage for an electrode is a function of its chemical characteristics and current density. See, W.J. Moore, Physical Chemistry, pp. 406-408 (Prentice Hall, 3rd ed. 1962). Current density is defined as the current applied per unit of actual surface area on an electrode. Techniques which increase the actual surface area of an electrode, such as acid etching or sandblasting the surface of the electrode, result in a corresponding decrease of the current density for a given amount of applied current. Inasmuch as the overvoltage and current density are directly related to each other, a decrease in current density yields a corresponding decrease in the overvoltage. The chemical characteristics of materials used to fabricate the electrode also impact overvoltage. For example, electrodes incorporating an electrocatalyst accelerate kinetics for electrochemical reactions occurring at the surface of the electrode.
It is known that certain platinum group metals, such as ruthenium, rhodium, osmium, iridium, palladium, platinum, and oxides thereof are useful as electrocatalysts. Electrodes may be fabricated from these metals, but more economical methods affix the platinum group metals to a conductive substrate such as steel, nickel, titanium, copper and so on. For example, U.S. Pat. No. 4,414,071 discloses coatings of one or more platinum group metals deposited as a metallic layer on an electrically-conductive substrate. Japanese Patent No. 9130/65-OPI application numbers 131474/76 and 11178/77, refers to use of a mixture of at least one platinum group metal oxide with a second metal oxide as a cathode coating. U.S. Pat. No. 4,238,311 teaches a cathode coating consisting of fine particles of platinum group metals, platinum group metal oxides or a combination thereof, affixed to a nickel substrate.
Industry has recently directed attention toward development of "zero-gap" electrolytic cells wherein an electrode, such as a cathode, is placed in contact with a membrane. This arrangement reduces the required overvoltage of prior "gap" cell designs by elimination of electrical resistance caused by electrolyte being disposed between the cathode and the membrane. In some zero-gap cells, it is advantageous to employ an extremely thin cathode to provide close contact between the cathode and the membrane and, thereby, fully utilize the advantage of the zero-gap cell design. A thin substrate also provides flexibility, which helps prevent damage to the membrane caused by contact with the cathode. However, use of a thin substrate presents problems in maintaining adherence of electrocatalytic coatings to the substrate. Substrates coated by prior methods can experience significant coating loss by decrepitation shortly after being placed in service, especially where the substrate is flexible. Accordingly, it is desirable to develop a coating resistant to loss during operation.
It is also difficult under prior methods to deposit an effective amount of catalyst on the electrode in a single application. Many prior methods, particularly those which prepare platinum group metal oxide coatings, require repeated applications to obtain an effective catalyst loading. In some methods, such as thermal oxidation of platinum group metal compounds placed on an electrode substrate, obtaining an effective amount of catalyst requires as many as eight or more separate applications of the compounds and subsequent thermal treatments. The cost associated with production of such electrodes is, in part, dependent upon labor costs. It is not surprising that such repeated applications greatly increase electrode production costs. A method which exhibits improved catalyst loading, i.e., one wherein an effective amount of catalyst may be applied in one application, would reduce such production costs.
It is, therefore, desirable to develop a method for producing novel low overvoltage cathodes having a coating that is tightly adhered to the underlying substrate and exhibiting improved electrocatalyst loading.